The invention relates to a semiconductor device for producing electromagnetic radiation and having a semiconductor body comprising a radiation-emitting element having an active layer, in which active layer electromagnetic radiation can be produced by injection of hot charge carriers, and an injection element, in which the said hot charge carriers are generated by avalanche multiplication, the radiation-emitting element and the injection element each having a monocrystalline epitaxial layer structure.
A semiconductor device of the kind described above is known from U.S. Pat. No. 4,631,731, FIG. 6.
Semiconductor devices for producing electro-magnetic radiation have been frequently used for several years in different fields of technology. They comprise an active semiconductor material of the socalled direct band transition type having mostly one or several radiation-emitting pn junctions and they can be used in devices whose radiation is not coherent and devices whose radiation is coherent. In the former case, these devices are generally designated as LED's (Light-Emitting Diodes), while in the latter case these devices are designated as lasers.
The radiation emitted by conventional semiconductor lasers generally has a wavelength of about 700 nm or larger. However, there is a great demand for lasers and LED's generating radiation of a shorter wavelength. More particularly this is the case4 with the use of these devices for optically reading-in and reading-out information ("digital optical recoding"=DOR). The attainable information density increases in inverse proportion to the square value of the wavelength of the radiation used. A further advantage is that at shorter wavelengths the optics used can be simpler.
In order to obtain LED's or lasers for such short wavelengths, for example from green to ultraviolet, use must be made of "direct" semiconductor materials having a large bend gap. The problem is that such materials, such as, for example, gallium nitride, zinc telluride, cadmium sulphide, zinc sulphide etc., in practice can be doped either only n-type or only p-type.
Therefore, it is substantially not possible to realize with these materials LED's or lasers by means of injection via a pn "homo" junction. With these materials, injection via pn hetero-junctions is in practice not possible either because substantially no "pairs" of semiconductor materials exist which simultaneously satisfy all necessary conditions, i.e. that:
1. sufficiently large band gaps, the materials having the smallest band gap being at the same time a "direct" semiconductor, are available; PA1 2. the possibility of opposite doping of both materials exists, and PA1 3. the lattice adaptation between both materials is sufficient.
Therefore, use must be made of other methods of injecting charge carriers. It is proposed in the aforementioned, U.S. Pat. No. 4631731 to inject for this purpose into the LED material or laser material "hot" charge carriers, i.e. electrons or holes having an energy larger than that corresponding to thermodynamic equilibrium, via an intermediate isolating layer. These hot charge carriers are supplied by a semiconductor cathode, which is separated by the isolating layer from the radiation-emitting element.
For various reasons, it is very difficult to realize such a device. In the first place, a very thin isolating layer would have to be used. The forbidden band width of an isolator is in fact so large that the injected hot charge carriers cannot pass this threshold or can pass it only with great difficulty. Further, also in connection with the fact that the structure is not monocrystalline, but consists of two parts each having its own crystal lattice mutually separated by a non-mono-crystalline isolating layer, the life of the injected hot charge carriers will be short, as a result of which the device cannot operate at all or cannot operate to the optimum.